From f50ad4b73e1bdf7fb40566650fa11bcb438ad5ce Mon Sep 17 00:00:00 2001 From: Joel Fernandes Date: Thu, 13 Feb 2025 11:47:27 -0500 Subject: [PATCH] srcu: Use rcu_seq_done_exact() for polling API poll_state_synchronize_srcu() uses rcu_seq_done() unlike poll_state_synchronize_rcu() which uses rcu_seq_done_exact(). The rcu_seq_done_exact() makes more sense for polling API, as with this API, there is a higher chance that there is a significant delay between the get_state..() and poll_state..() calls since a cookie can be stored and reused at a later time. During such a delay, if the gp_seq counter progresses more than ULONG_MAX/2 distance, then poll_state..() may return false for a long time unwantedly. Fix by using the more accurate rcu_seq_done_exact() API which is exactly what straight RCU's polling does. It may make sense, as future work, to add debug code here as well, where we compare a physical timestamp between get_state..() and poll_state() calls and yell if significant time has past but the grace period has still not progressed. Reviewed-by: Neeraj Upadhyay Reviewed-by: Paul E. McKenney Reviewed-by: Kent Overstreet Cc: Kent Overstreet Signed-off-by: Joel Fernandes --- kernel/rcu/srcutree.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 9a59b071501bd..48047260697ec 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -1589,7 +1589,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu); bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie) { if (cookie != SRCU_GET_STATE_COMPLETED && - !rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, cookie)) + !rcu_seq_done_exact(&ssp->srcu_sup->srcu_gp_seq, cookie)) return false; // Ensure that the end of the SRCU grace period happens before // any subsequent code that the caller might execute. -- 2.47.2